The present invention relates to a magnetic recording medium, comprising a magnetic layer having ferromagnetic fine particles and a binder dispersed in it and placed on a non-magnetic support member, whereby said magnetic recording medium has very high electromagnetic transfer characteristics and high durability.
The magnetic recording medium is widely used in tape for tape recording, video tape, floppy disk, etc. In the magnetic recording medium, a magnetic layer where ferromagnetic powder is dispersed in a binder is deposited on a non-magnetic support member.
The magnetic recording medium must be at high level in various characteristics such as electromagnetic transfer characteristics, running durability and running performance. Namely, an audio tape for recording and reproducing musical sound must have higher ability to reproduce original sound. A video tape must have excellent electromagnetic transfer characteristics such as high ability to reproduce original image. In particular, a magnetic recording medium for digital video tape recorder, which must be suitable for high density recording, must have higher electromagnetic transfer characteristics.
In addition to the excellent electromagnetic characteristics as described above, the magnetic recording medium must have good running durability as already described. To ensure good running durability, abrasive and lubricant are added in the magnetic layer.
However, to obtain good running durability by the use of abrasive, it is necessary to increase the quantity of the added abrasive to some extent. As a result, filling degree of the ferromagnetic powder decreases. In case an abrasive material having larger particle size is used to obtain good running durability, the abrasive material is more likely to protrude or to produce roughness on the surface of the magnetic layer. Therefore, the improvement of running durability by the use of abrasive often leads to deterioration of the electromagnetic transfer characteristics.
When it is tried to improve the running durability by the use of lubricant, it is necessary to increase the quantity of the added lubricant. As a result, the binder is more likely to be plasticized, and the durability of the magnetic layer tends to decrease.
In particular, in a device using magnetic recording medium for high density recording, the number of revolutions of magnetic head is increased. In a digital video tape recorder, the number of revolutions of magnetic head is 9600 rpm, and this is extremely higher when compared with that of a recorder for home use (1800 rpm) or that of a recorder for business use (5000 rpm). In this respect, higher durability is required for this purpose. Above all, there are strong demands on the improvement of anti-abrasive property of the medium against friction on a rotating head in still operation.
Also, to improve the durability and the electromagnetic transfer characteristics, the binder, serving as a major component of the magnetic layer, also plays an important role. The materials used in the past such as vinyl chloride resin, cellulose resin, urethane resin, acrylic resin, etc. are low in anti-abrasive property of the magnetic layer, and this often results in contamination of running components of the magnetic tape.
To overcome the above problems, a method to use hard binder and to increase hardness of the magnetic layer is adopted.
For example, JP(A) 59065931 discloses a magnetic recording medium, which uses diester of carboxylic acid with neopentyl glycol, trimethylolpropane, pentaerythritol, etc. and denatured ester of ethylene oxide. However, the magnetic recording medium of this type is insufficient in repeated running durability when it is applied for high density recording as currently performed, and problems such as head contamination, output decrease, etc. arise.
JP(B) 6618063 describes a magnetic recording medium, which comprises a binder of a compound having polyurethane bond and a magnetic layer where diester of carboxylic acid with dihydric alcohol is added. Also, JP(B) 7214648 describes a magnetic recording medium using diester of aliphatic dicarboxylic acid. JP(A) 5680829 discloses the use of aliphatic dibasic acid having a specific chemical structure as lubricant, and JP(A) 59028236 discloses the use of dibasic acid ester having a specific number of carbon atoms as lubricant. Further, JP(A) 59186129 describes a magnetic recording medium where dioleyl maleate is present in the surface.
However, the lubricating materials disclosed in the prior art as described above are disadvantageous in that resistance to hydrolysis is low and storage stability is inferior (e.g. decrease of durability) when it is stored for long time under high temperature and high humidity conditions. Also, when these lubricating materials are applied on a tape for high density recording as currently performed or in a tape for digital video tape recorder, repeated running durability and still durability are not satisfactory.
To solve the above problems, it is an object of the present invention to provide a magnetic recording medium, which has high repeated running durability, high still durability and excellent running durability at high temperature and high humidity conditions and high storage stability under high temperature and high humidity conditions, and also to provide a magnetic recording medium suitable for high density recording and for digital video tape recorder.